Fiberglass Repair
by Don Casey
Few things are more disheartening to the boatowner than staring at the
fuzzy edge of broken fiberglass. However, the repairability of fiberglass
is one of its best characteristics. The most horrifying hole in a fiberglass
hull is quickly healed with a bit of glass fabric, a supply of resin,
and equal parts skill and care. And the repair is less patch than graft-a
new piece of skin indistinguishable from the old.
Fiberglass lay-up is nothing more than layers of glass fabric saturated
with polyester (or epoxy) resin, yet most boatowners imagine a self-applied
repair as only slightly more durable than a wet Band-Aid. This is a
false concern. Follow a few simple rules and your lay-up will be just
as durable as the rest of the boat.
Cut
Away the Damage
Impact damage nearly always results in some associated delamination.
Tap the impact area with the end of a plastic screwdriver handle to
determine the extent of the damage; solid laminate sounds sharp, delamination
dull. Check inside the boat to make sure nothing is in the way, then
make a circular or oval cut to remove the damaged area. Never try to
save damaged fiberglass; always cut it out and replace it with new laminate.
Check all the edges and enlarge the hole if you find any additional
delamination.
Working
from Inside
If the damage area is small and above the waterline, make the repair
from inside the hull, if possible. You are going to bevel the edge of
the hole with a 12-to-1 chamfer, so if you repair a 3-inch diameter
hole through a 1/2-inch-thick hull from the outside, you end up with
about 15 inches (diameter) of surface damage to refinish. Repair it
from the inside and you have only a 3-inch hole to refinish.
A second reason to make the repair from the inside is that you can back
the hole on the outside with a polished surface, creating a mold that
allows you to lay-up the repair the same way the boat was built-gelcoat
first. Very little finish work will be required.
Dewax
Before grinding, always wash the area around the hole thoroughly with
a dewaxing solvent. The original fiberglass will have traces of mold
release on the outer surface and wax surfactant on the inner surface.
If you fail to remove the wax first, grinding will drag it into the
bottom of the scratches and weaken the bond.
Grind
During the lay-up process, because each layer is applied before the
previous one fully cures, each application of resin links chemically
with the previous one to form a solid structure-as though all the layers
were saturated at once. Unfortunately, no matter how strong the laminate-to-laminate
bond, the initial bond of any repair is mechanical, not chemical. Consequently,
grinding is the key to getting a strong repair.
Use a disk sander loaded with a 36-grit disk to grind a 12-to-1 bevel
around the perimeter of the hole inside. Also grind a rectangular area
of the inner surface a few inches beyond the bevel to accommodate a
finishing layer of cloth. Protect your eyes with goggles and your lungs
with a good dust mask. Long sleeves will reduce skin irritation. Tilt
the sander so that only one side of the disk is touching the surface
and the dust is thrown away from you. After you brush away the dust
and wipe the area with an acetone-dampened rag, the sanded surface should
have a uniform dull look.
Mask
and Mold
To prevent any resin runs from adhering, give the exterior surface of
the skin around the hole a heavy coat of paste wax, taking care not
to get any on the edge or inside the hole. Mask the area below the hole.
Cut a scrap of smooth plastic laminate (Formica) or thin clear acrylic
(Plexiglas) a foot larger than the hole. Wax this backer, then spritz
it with polyvinyl alcohol (PVA) mold release. Screw or tape the backer
to the outer surface. If the hull is flat or curving in only one direction
in the damage area, the backer will assume the correct curve. If the
hull is spherical, i.e. curving in two directions, acrylic screwed to
the hull can sometimes be coaxed into the correct shape if warmed with
a heat gun (before applying the mold release).
Fabric
Cut the fiberglass fabric to fit the hole. Unless you have reason to
follow a different schedule, begin with two layers of 1 1/2-ounce mat,
then alternate mat and 6-ounce cloth. The number of laminates will be
determined by the thickness of the hull; you will roughly need one layer
for every 1/32 inch. Cut the first layer of mat 1 inch larger than the
hole, i.e., overlapping the bevel by 1/2 inch all around. Subsequent
pieces should be 1/2 inch larger all around than the previous one.
Using
Polyester or Vinylester Resin
For above-the-waterline repairs you can use either polyester or vinylester
resin. Of course, for an even stronger repair you can also use epoxy,
but not if the surface of the repair will be gelcoat. (You should use
epoxy for underwater repairs.)
If you are doing your repair with polyester or vinylester resin, you
need laminating resin. Laminating resin does not fully cure while exposed
to air, which allows you to get a chemical bond between the multiple
laminates you will be applying. To get the final laminate to cure, you
simply seal it from the air, either with a plastic or by coating it
with polyvinyl alcohol (PVA) mold release.
Catalyst
The catalyst for both polyester and vinylester resin is methyl ethyl
ketone peroxide, or MEKP. Do not confuse MEKP with the common solvent
MEK; they are not the same.
Polyester resin usually requires 1 to 2 percent of hardener by volume
(follow the manufacturer's instructions). As a rule of thumb, four drops
of hardener will catalyze 1 ounce of resin at 1 percent. Be certain
to stir the catalyst in thoroughly or part of the resin will be undercured,
weakening the lay-up.
You can adjust the cure time by adding more or less catalyst. Temperature,
weather, and the thickness of the laminate all affect curing times.
Some experimentation is generally required. The mix shouldn't kick (start
to harden) in less than 30 minutes. Hardening in about two hours is
probably ideal, but overnight is just as good unless the wait will hold
you up. Always err on the side of too little catalyst; if you add too
much, the resin will "cook," resulting in a weak lamination.
Gelcoat
Gelcoat is essentially pigmented polyester resin. Start the repair by
spraying or brushing about 20 mils of color-matched gelcoat onto the
waxed backer. Check the gelcoat thickness with a toothpick; 1/32 is
about 30 mils.
Lay-up
When the gelcoat kicks, wet it with polyester resin and lay-up the first
two layers of mat and one layer of cloth, compressing them against the
gelcoat and working out all voids and bubbles with a resin roller and/or
a squeegee.
Let the first three plies kick, then lay up four additional plies. Never
lay up more than four plies at a time or the generated heat may "cook"
the resin and weaken it. Continue the lay-up four plies at a time until
the repair is flush with the interior surface.
Finish
For a finished look, cut a rectangular piece of mat and one slightly
larger of cloth and apply these over the patch, smoothing them with
a squeegee. Seal this top layer with plastic or PVA to allow a full
cure.
Remove the backer from the exterior surface. Fill imperfections in the
new gelcoat with gelcoat paste and allow it to cure fully. Clean the
area around the patch, then sand-if necessary-and polish the repair
area.
For more
information about repairing fiberglass, consult Sailboat Hull &
Deck Repair by Don Casey.
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